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Attention Passengers: Please Keep Your Shoes On

Soon you could be spared one of the most annoying aspects of air travel: removing your shoes in the security line. Researchers have developed a hand-held sensor that can quickly detect the presence of triacetone triperoxide (TATP), the explosive of choice in footwear. The device also shows promise for sniffing out a host of other toxic and dangerous chemicals.

Discovered in 1895 by German chemist Richard Wolffenstein, who pioneered the use of peroxide as an explosive, TATP has no other known use except as an explosive. Richard Reid, the convicted "shoe bomber," introduced the stuff to the popular consciousness when he customized the soles of his shoes (inset) with TATP and tried to ignite them aboard a flight from Paris to Miami in 2001. Ever since, almost all U.S. airline passengers have had to remove their shoes and submit them to x-ray examination before boarding so the scanners can spot the denser explosive material.

The reason for the cumbersome screening method is that TATP is notoriously difficult to spot. It doesn't glow in ultraviolet light like some other explosives, and it can be detected directly only with elaborate instruments, and even then the detection process can be clouded by humidity in the air or by personal hygiene products or perfumes. Adding to the threat, TATP is easy for terrorists to manufacture, requiring only commonly available chemicals.

But TATP has an Achilles' heel: it emits trace amounts of vapor. Enter chemist Kenneth Suslick of the University of Illinois, Urbana-Champaign. For the past decade, he has been working on a class of sensors called colorimetric arrays that can detect toxic substances at low concentrations and distinguish between varieties of the same substance. For example, Suslick says, his son Ben was able to use one of his arrays in a high school chemistry experiment to differentiate between brands of coffee.

The sensors are dots of chemicals deposited on a plastic square that change color when exposed to even the tiniest amounts of specific substances. As Suslick and lead author Hengwei Lin reported online this month in the Journal of the American Chemical Society, the key to detecting TATP is an acid catalyst called Amberlyst 15. "We flow the air containing TATP vapor over [the catalyst]," Suslick explains. When TATP vapor comes in contact with the catalyst, Amberlyst 15 instantly breaks down the TATP into acetone and peroxide, both of which alter the colorimetric array's colors in predictable patterns that can be instantly compared with an image database (see picture).

When the researchers wafted TATP vapors across the array, they found they could detect TATP in concentrations of two parts per billion within seconds—making it sensitive enough to pick up explosives concealed in shoes. The hand-held device the two researchers developed (see picture) works the same way. Security officials would wave it near a passenger's shoes. If the device's digital image processor reads color changes that match the onboard database, it confirms the presence of TATP.

"This is a general-purpose technology," Suslick says, explaining that in previous work, he and colleagues have demonstrated that they could "easily detect and identify 20 of the most hazardous [toxic industrial gases] immediately dangerous to life or health," such as fluorine or hydrogen sulfide, and in concentrations well below their permissible exposure levels. Suslick and Lin are also attempting to miniaturize the device to the point it becomes "the chemist's equivalent of a radiation badge," a wearable sensor that can monitor a person's exposure to toxic chemicals. But for the most immediate application, airport security, the sensor manufacturer iSense of Santa Monica, California, is commercializing the new device. "Imagine not having to take your shoes off," Suslick says.

"This is a real advance," say organic chemist Nathaniel Finney of the University of Zurich in Switzerland. It "represents a significant departure from the traditional approach to molecular recognition, in which chemists try to design a single molecule that will interact with a single [chemical] of interest." He says the requirements for implementing the technology "are relatively minimal, and the reported limits of detection are remarkable."

Chemist Robert Grubbs of the California Institute of Technology in Pasadena calls the work "a very practical approach to an important problem." Grubbs, who won the 2005 Nobel Prize in chemistry, adds that the TATP detector hasn't been "hindered by issues of humidity and the presence of other chemicals in the environment, as have past approaches."